Fly ash is a by-product of burning coal, typically generated during the production of electricity at coal-fired power plants. Fly ashes are mainly composed by aluminosilicates partially vitrified, as well as mineral phases such as quartz, hematite, maghemite, anhydrite and so on which had been present as impurities in the original coal.
ASTM C 618-85 (“Standard specification for fly ash and raw calcinated natural pozzolan for use as a mineral admixture in Portland cement concrete”) has classified fly ash into two classes, Class C and Class F, depending on the total sum of silica, alumina and ferric oxide present. Class F contains more than 70% of the above oxides and Class C contains less than 70% but more than 50%. Class F fly ash is typically low in calcium oxide (<8%) whereas Class C has a higher content being sub-classified in two categories: Class CI (8-20% CaO) and Class CH (>20% CaO). Therefore, Class F fly ash is not usually considered as a cementitious material by itself because, due to its low calcium oxide content, it cannot be agglomerated after hydration to produce bonding strength in the final product, contrary to Class C fly ash.
Fly ash is a by-product that has to be used and consumed to reduce its environmental impact. Nowadays, it has mainly been used as a partial substitute in Ordinary Portland Cement due to its pozzolanic reactivity. However, there is a limitation in the replaced quantity because the pozzolanic reaction rate is very low at room temperature causing initial low strength and fast neutralization.
Recently trials have been carried out to increase the pozzolanic reaction rate by using activators such as alkaline and alkaline earth compounds (ROH, R(OH)2), salts from weak acids (R2CO3, R2S, RF) and silicic salts type R2O(n)SiO2, where R is an alkaline ion from Na, K or Li. However, either the activation efficiency is not enough or there are some undesired interactions between ordinary Portland cement and activators, which causes rheological and/or mechanical problems. This fact promotes the use of additional components, mainly admixtures, which increases the complexity of the formulation and makes worse the technological development of these products.
The high amount of lime CaO in fly ash type C provides the waste product with intrinsic cementitious properties. On the other hand fly ash type F does not by itself develop any strength on hydration, and an activation of the product is requested to ensure that strength development will take place on hydration. A major advantage to prefer fly ash type F rather than fly ash type C is the high availability in large quantities of fly ash type F and its lower market price. Since transportation costs of industrial wastes would be a key issue for the cost effectiveness of the final product or binder, the selection of fly ash type F is guided by its availability in large quantities and its dense geographic distribution.
For many years, many formulations and processes have been proposed to activate fly ash or industrial wastes in order to use it as a cementitious material.
U.S. Pat. Nos. 5,435,843 and 5,565,028 described the activation of Class C fly ash at room temperature with strong alkali (pH>14.69) to yield cementitious properties. Even though there is no express mention of Class F fly ash use in these patents, the cement containing Class C fly ash according to these patents has limited application due to the corrosive properties (pH>14.6) of the used activators.
Patent EP0858978 discloses that high volumes of activated Class C Fly ash (>90%) may be used as a cementitious binder. The binder contains a mix of Class C and Class F Fly ashes wherein the dosage of Class F fly ash has to be limited up to 60% due to its low reactivity. In this case, Class F Fly ash is mentioned but it is used together with clinker and admixtures like citric acid, borax, Boric acid, which are very expensive, and KOH, which is corrosive (pH>13). Furthermore, formulations get complex because the high number (>6) of presented components.
In a similar way, U.S. Pat. No. 5,482,549 describes a cement mixture containing at least 2% by weight of Portland cement clinker, 2-12% by weight of sodium silicate, fly ash and blast furnace slag. The patent specifies that the fly ash has to be ground to a specific surface of more than 500 square meters per kg which is very important and yields high manufacturing costs (energy consumption, handling, etc.). Furthermore, this document doesn't mention the use of Class F fly ash.
Xu et al. “The activation of Class C-, Class F-Fly Ash and Blast Furnace Slag Using Geopolymerisation”, 8th CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag and natural Pozzolans in Concrete, Las Vegas, Calif., USA (2004), shows that Class F fly ash can only be properly activated when using a highly alkaline soluble silicate solution. Following this line, patent U.S. Pat. No. 5,601,643 proposes an invention related with chemically-activated fly ash cementitious materials, preferably Class F Fly ash, where high content of alkali metal and/or alkaline earth metal silicate are used to obtain high strength cementitious mixtures. However, this invention has a limited application because: 1) a high curing temperature is need, 2) a high pH (>14, corrosive products) is required and therefore, safety conditions are necessary to handle the mixture and 3) the cost of the mixture is high due to the high quantities of soluble silicates and alkalis used. Furthermore, this patent proposes to use activator like alkali hydroxides, which cannot be employed in a solid state because of theirs affinity to moisture, carbonation and huge heat released during dissolution. Therefore, the cementitious material according to this invention can't be transport in a powder form. Furthermore, formulations related with high alkalis content and high pH cause alkali-leaching problems and efflorescence due to the overdose of activators. The overdose of activators is due to Class F fly ash that is considered as a binder and not as active filler, which requires less alkaline dosage for being activated.
U.S. Pat. Nos. 6,572,698 and EP1091913 are related to an activated supersulphated aluminosilicate binder containing aluminosilicates, calcium sulphate and activator, which contains alkali metal salts. Fly ash is mentioned as aluminosilicate, but it is not specified the Class of fly ash used. Binders with more than 90% of fly ash were activated at room temperature using cement kiln dust (CKD) and admixtures such as plasticizers and accelerators. Shah et Wang. “Development of “Green cement” for sustainable concrete using Cement Kiln Dust (CKD) and Fly Ash”, International Workshop on Sustainable Development and Concrete Technology, (Beijing, China, May 20-21, 2004), shows that Class F fly ash activated with CKD at room temperature provides binders with low initial and final mechanical strength. Therefore, fly ash from patents U.S. Pat. Nos. 6,572,698 and EP1091913 has to be either class C or a mix of the mentioned one and Class F. Thus, it is not possible to activate high volumes of Class F Fly ash with CKD. Furthermore, binders made from CKD are not totally environmentally friendly because CKD is a by-product from the cement industry. In addition, the binder formulations were complex due to a high number of components. Thus, these products are expensive and their technological development is worse.
An example of this fact is the binder proposed by these patents made of fly ash, blast furnace slag, anhydrite, CKD, accelerators and plasticizers. In this case, the proportion of the fly ash does not exceed 40%. Furthermore, CKD is a by-product from cement industry which quality, mainly measured from the quantity of alkalis, depends on the variations of raw materials from cement production.
Patent WO9831644 concerns a method to manufacture cheap geopolymeric cement using alkaline aluminosilicates from geologic origin. In this case, the hardener is made of blast furnace slag and metakaolin. However, although the inventor may achieve a cost reduction from activators, the binder has limited application because of the high cost of metakaolin which is a product coming from calcined kaolin, which is not a residue. Furthermore, metakaolin is a product with small density and high water demand because of its high specific surface. This fact may produce a higher water demand of the binder, which is counterproductive for the mechanical properties.
It can thus be seen that a formulation based on high volumes of alkaline activated class F fly ash residue (>60%), complying with industrial requirements, involving a limited number of components would be of considerable advantage for the construction industry to provide a multi-purpose binder compared to the disclosed solutions hereinabove.
The aim of the invention is to remedy to the above drawback by providing a binder, which have the following characteristic:                environmental friendly        easy to formulate involving limited number of components        safe and easy to handle and to prepare with conventional equipment.        multipurpose, versatility to be used in bags, in bulk, as a ready mix, for all type of mortars and concrete applications        cost effectiveness        ability to be stored over long period of time        ability to be prepared on the construction site        no specific curing conditions        
Typically, the invention doesn't aim to use any cement or cement related compounds (like cement kiln dusts for instance). The advantage not to use cement in the formulation of the binder is mainly based on the objective of simplicity and polyvalence of the invention. Cement or the like additions in the formulation will lead to additional problems of interactions with the chemical activators that need to circumvent by further specific chemicals etc, special curing conditions, high pH etc. The objective of early strength development, as well as the universal property of the binder will be very difficult to achieve. Finally, the ecological advantages of the product according to the invention will be reduced since cement, clinker or cement kiln dust additions are correlated to additional CO2 emissions.
It will be seen in the following description that none of the prior art present the technical features and none of the prior art have all advantages provided by the present invention.